[WIP] support external blob

[imeoer]

Related PR: containerd/nydus-snapshotter#580

Relevant Issue (if applicable)

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Details

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Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Attention: Patch coverage is 41.28920% with 337 lines in your changes are missing coverage. Please review.

Project coverage is 61.17%. Comparing base (d12634f) to head (5e6cdba).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1545      +/-   ##
==========================================
- Coverage   61.40%   61.17%   -0.24%     
==========================================
  Files         144      148       +4     
  Lines       46962    47428     +466     
  Branches    44498    44964     +466     
==========================================
+ Hits        28837    29012     +175     
- Misses      16646    16920     +274     
- Partials     1479     1496      +17     

Files	Coverage Δ
builder/src/core/bootstrap.rs	77.63% <100.00%> (+0.29%)	⬆️
builder/src/core/chunk_dict.rs	69.93% <ø> (ø)
builder/src/lib.rs	64.66% <ø> (ø)
builder/src/merge.rs	70.79% <100.00%> (ø)
builder/src/stargz.rs	74.81% <100.00%> (+0.14%)	⬆️
builder/src/tarball.rs	63.40% <100.00%> (+0.56%)	⬆️
storage/src/backend/mod.rs	56.07% <ø> (ø)
storage/src/device.rs	69.13% <100.00%> (+0.09%)	⬆️
storage/src/meta/chunk_info_v1.rs	95.90% <100.00%> (ø)
utils/src/filemap.rs	94.32% <100.00%> (-0.23%)	⬇️
... and 19 more

... and 2 files with indirect coverage changes